The present invention relates generally to a reactor tube for use in a high temperature, fluid-wall reactor in which substantially all of the heat is supplied by radiation coupling, and which is useful for carrying out many chemical reactions which heretofore have been impractical or only theoretically possible.
In particular it is contemplated that the present reactor tube be utilized in a reactor which utilizes radiation coupling as a heat source, maintains the contemplated chemical reactions in isolation within a protective fluid blanket or envelope out of contact with the containing surfaces of the reactor tube, and which includes a heat shield which substantially encloses the radiant energy heating means and the reaction zone to define a black body cavity. As used herein, the term "black body cavity" is generally intended to denote a space which is substantially enclosed by a surface or surfaces which function as a heat shield and from which, ideally, no radiation can escape.
High temperature reactors are presently employed to carry out pyrolysis, thermolysis, disassociation, decomposition and combustion reactions of both organic and inorganic compounds. Substantially all such reactors transfer heat to the reactants by convection and/or conduction, but this characteristic inherently produces two major problems which limit the nature and scope of the reactions which may be carried out. Both problems result from the fact that in a conventional reactor, which transfers heat to the reactants by convection, the highest temperature in the system is necessarily at the interface between the inside wall of the reactor and the reactant stream.
The first problem involves the limitations on available temperatures of reaction which are imposed by the strength at elevated temperature of known reactor wall materials. The decreasing capability of such materials to maintain their integrity under conditions of increasing temperature is, of course, well known. Since it is necessary that such materials be heated in order for thermal energy to be transferred to the reactant stream, available reaction temperatures have been limited by the temperature to which conventional reactor walls may be safely heated. This factor is particularly critical in cases where the contemplated reaction either must take place at or produces high pressures.
The second problem inherently results both because the wall of a conventional reactor is at the highest temperature in the system and because convective/conductive heat transfer requires contact between the wall and the reactant stream. Being at such elevated temperature, the reactor wall is an ideal if not the most desirable reaction site in the system and, in many instances, reaction products will accumulate and build up on the wall. Such build-up impairs the ability of the system to transfer heat to the reactants and this ever increasing thermal impedance requires the source temperature to be raised progressively just to maintain the initial rate of heat transfer into the reactant stream. Obviously, as the build-up increases, the required source temperature will eventually exceed the temperature capabilities of the reactor wall material. Moreover, as additional energy is required to sustain the reaction, the process becomes less efficient in both the technical and economic sense. Thus, at the point where the contemplated reaction can no longer be sustained on the basis of either heat transfer, strength of materials, or economic considerations, the system must be shut down and cleaned.
Usually, cleaning is peformed mechanically by scraping the reactor wall or chemically by burning off the deposits. In some continuous processes, it has been attempted to scrape the reactor wall while the reaction proceeds. However, the scraping tool itself necessarily becomes a reaction site and, thereafter, must be cleaned. In any event, this down time represents a substantial economic loss. In many instances, a second system will be installed in order to minimize lost production time. However, such additional equipment generally represents a substantial capital investment. Some high temperature chemical reactors include a tube which is heated to a temperature at which its inner walls emit sufficient radiant energy to initiate and sustain the reaction. However, as in the case of conductive and convective reactors, for reactions yielding solid products there is frequently an undesirable build-up of product on the tube walls which leads to reduced heat transfer and even clogging of the tube.
The apparatus for the manufacture of carbon black disclosed in U.S. Pat. No. 2,062,358 includes a porous tube disposed within a heating chamber. Hot gas is directed from a remote furnace into the chamber, and thereafter forced through the wall of the porous tube to mix with the reactants. Thus, only convective transfer of heat from a fluid to reactants is employed. This, together with the absence of a "black body cavity" necessitates the flow of a large volume of fluid through the heating chamber in order to make up for heat losses.
U.S. Pat. No. 2,769,772 discloses a reactor for heat-treating fluid materials such as hydrocarbons which includes two concentric tubes disposed in a flame heated furnace. Reactants flow axially through the pervious inner concentric tube. A heat-carrier gas flowing in the annular chamber between the concentric tubes is heated by contact with the outer wall. Fluids in the inner tube are heated by convection when the heat-carrier gas passes through the pervious wall and mixes with them. Radiant heat transfer is expressly avoided. In fact, it is impossible to heat the inner tube without simultaneously heating the outer tube to at least as high a temperature.
U.S. Pat. Nos. 2,670,272; 2,670,275; 2,750,260; 2,915,367; 2,957,753; and 3,499,730 disclose combustion chambers for producing pigment-grade titanium dioxide by burning titanium tetrachloride in oxygen. In the '275 patent, which is representative of this group of references, titanium tetrachloride is burned in a porous, refractory tube. An inert gas is continuously diffused through the porous tube into a combustion chamber where it forms a protective blanket on the inner surface of the tube. This gaseous blanket substantially reduces the tendency of the titanium dioxide particles to adhere to the walls of the reactor. Since the combustion of titanium tetrachloride is an exothermic reaction, no provision is made to supply heat to the reaction mixture as it passes through tube. In fact, the '275 patent teaches that it is advantageous to remove heat from reactor chamber either by exposing the porous tube assembly to the atmosphere or by circulating a cooling fluid through a coil disposed about the porous tube.
The high temperature chemical reactor disclosed in application of Edwin Matovich, Ser. No. 271,560, filed July 13, 1972, now U.S. Pat. No. 3,933,434 entitled "High Temperature Chemical Reactor" overcomes the problems which had been encountered, but the porous, refractory reactor tube specifically described, being of monolithic construction, is subject to serious size limitations since the state-of-the-art is such that suitable porous refractory materials cannot be cast or machined economically in sizes large enough for large-scale operation. For example, at present, porous carbon tubes cannot be cast economically into tubes larger than about four feet in length and six inches in diameter. Porous ceramic tubes are also subject to such size limitations. Nor is it possible to circumvent these size limitations by constructing a large porous refractory tube from a series of rings stacked one on top of another because the nonuniformity of the flow of inert fluid at the joints between the rings will lead to carbon deposits near the joints.